14.2.12 InnoDB Multi-Versioning

InnoDB is a
multi-versioned storage engine:
it keeps information about old versions of changed rows, to
support transactional features such as concurrency and
rollback. This information is
stored in the tablespace in a data structure called a
rollback segment
(after an analogous data structure in Oracle).
InnoDB uses the information in the rollback
segment to perform the undo operations needed in a transaction
rollback. It also uses the information to build earlier versions
of a row for a consistent
read.

Internal Details of Multi-Versioning

Internally, InnoDB adds three fields to each
row stored in the database. A 6-byte DB_TRX_ID
field indicates the transaction identifier for the last
transaction that inserted or updated the row. Also, a deletion is
treated internally as an update where a special bit in the row is
set to mark it as deleted. Each row also contains a 7-byte
DB_ROLL_PTR field called the roll pointer. The
roll pointer points to an undo log record written to the rollback
segment. If the row was updated, the undo log record contains the
information necessary to rebuild the content of the row before it
was updated. A 6-byte DB_ROW_ID field contains
a row ID that increases monotonically as new rows are inserted. If
InnoDB generates a clustered index
automatically, the index contains row ID values. Otherwise, the
DB_ROW_ID column does not appear in any index.

Undo logs in the rollback segment are divided into insert and
update undo logs. Insert undo logs are needed only in transaction
rollback and can be discarded as soon as the transaction commits.
Update undo logs are used also in consistent reads, but they can
be discarded only after there is no transaction present for which
InnoDB has assigned a snapshot that in a
consistent read could need the information in the update undo log
to build an earlier version of a database row.

The physical size of an undo log record in the rollback segment is
typically smaller than the corresponding inserted or updated row.
You can use this information to calculate the space needed for
your rollback segment.

In the InnoDB multi-versioning scheme, a row is
not physically removed from the database immediately when you
delete it with an SQL statement. InnoDB only
physically removes the corresponding row and its index records
when it discards the update undo log record written for the
deletion. This removal operation is called a
purge, and it is quite fast,
usually taking the same order of time as the SQL statement that
did the deletion.

If you insert and delete rows in smallish batches at about the
same rate in the table, the purge thread can start to lag behind
and the table can grow bigger and bigger because of all the
“dead” rows, making everything disk-bound and very
slow. In such a case, throttle new row operations, and allocate
more resources to the purge thread by tuning the
innodb_max_purge_lag system
variable. See Section 14.12, “InnoDB Startup Options and System Variables” for more
information.